1. Field of the Invention
This invention relates to molten carbonate fuel cell power plants.
2. Description of the Prior Art
Molten carbonate fuel cells are well know in the art. These cells comprise an ion conducting molten carbonate electrolyte sandwiched between an anode and cathode electrode. Cells of this type are somewhat more fully described in commonly owned U.S. Pat. Nos. 3,878,296 Vine et al. and 3,615,839 Thompson et al.
The design of any commercial power plant has as one of its major considerations the cost per kilowatt of electricity produced. Cost per kilowatt includes both capital expense and operating expense. Obviously, it is extremely important that the fuel cells operate efficiently, but the cost for obtaining high efficiency cannot be excessive. Cell efficiency is directly tied to the suitability of the fuel and oxidant compositions supplied to the anode and cathode, respectively. The anode reaction in a molten carbonate fuel cell is shown by the following equation: EQU CO.sub.3 .sup.= + H.sub.2 .fwdarw. CO.sub.2 + H.sub.2 O + 2e.sup.-
The cathode reaction is shown by the following equation: EQU 1/2 O.sub.2 + CO.sub.2 + 2e- .fwdarw. CO.sub.3.sup.=
the unique difference between the anode and cathode reactions of molten carbonate fuel cells as compared to the anode and cathode reactions of more conventional acid or base electrolyte fuel cells is that the molten carbonate cells consume carbon dioxide at the cathode and produce carbon dioxide at the anode.
Good performance (high current density at high cell voltage) in a molten carbonate fuel cell necessarily requires reasonably high partial pressures of both carbon dioxide and oxygen at the cathode. It is generally agreed that a practical molten carbonate fuel cell system requires the carbon dioxide produced at the anode be returned to the cathode since the air used as the oxidant at the cathode does not naturally contain enough carbon dioxide for efficient performance. The aforementioned Thompson et al patent discloses one technique for accomplishing this task: transferring the CO.sub.2 in the anode exhaust to the cathode inlet air stream by means of a membrane selectively permeable to CO.sub.2. Another technique is to combine the entire anode exhaust with the cathode inlet air stream, such as shown in U.S. Pat. No. 3,436,271 Cole et al. Ideally, it is desirable to have twice as much CO.sub.2 as O.sub.2 in the oxidant stream since two moles of CO.sub.2 are consumed for each mole of O.sub.2.
Another important consideration in the design of commercially feasible molten carbonate fuel cell power plants is cell cooling. Molten carbonate cells typically operate at temperatures of about 1200.degree. F. Considerable quantities of heat are produced during operation which must be removed to prevent overheating of the cells. Cooling the cells in an economical manner can make the difference between commercial success and failure.
One method of cooling a plurality of molten carbonate fuel cell stacks is to use the process air stream for cooling as well as for delivery of the cathode side fuel cell reactants. With this method the air, enriched with CO.sub.2 from the anode exhaust, is divided into equal portions and passes in parallel through the cathode side of the stacks. This cooling method is hereinafter referred to as single pass process air cooling. (Process air, as that term is used herein, simply means the air used in the electrochemical process.) Single pass process air cooling has the drawback that, for a system with "n" stacks, the process air stream for each stack is limited to receiving only 1/n times the amount of CO.sub.2 produced by the stacks. Diluting this amount of CO.sub.2 with the necessary amount of air for cooling results in CO.sub.2 partial pressures which are often less than satisfactory, resulting in a power plant which may not be commercially attractive.
From a fuel cell performance point of view, the best method for cooling the fuel cells is with a cooling stream which passes through the cells but is separate from the process air stream. This permits the lowest possible air flow rate at the cathode since the process air is not needed for cooling; CO.sub.2 partial pressure is therefore maximized. This method, however, requires separate cooling fluid passageways within the stack, which adds considerable expense to the stacks and is a complex technical problem.